Flexible membrane diffusers have been used in the diffusion of gases into liquids, such as in the aeration of wastewater. The flexible membranes have been used with tubular and disc type diffusers. Examples of each a tubular membrane diffuser and a disc type membrane diffuser are provided in U.S. Pat. No. 7,044,453 to Tharp.
Flexible membrane diffusers are conventionally constructed of rubber or a similar material which is punctured to provide a large number of perforations. It is known that, in the aeration of wastewater, the highest efficiency is achieved when the gas is released as fine bubbles. When gas is applied to the diffuser, the gas pressure expands the membrane away from a diffuser body and causes the perforations to open so that gas discharges through them in the form of fine bubbles. When the gas pressure is relieved, the membrane collapses on the diffuser body to close the perforations and prevent the liquid from entering the diffuser.
Although flexible membrane diffusers are advantageous in many respects and have achieved widespread acceptance in a variety of gas diffusion applications, they are not wholly free of problems. In a wastewater treatment application, materials in the liquid can become deposited on and build up on the membrane to clog or partially clog the perforations and thus reduce the efficiency of the diffuser. For example, fats, greases and other substances which are commonly found in wastewater can adhere to the membrane. Other substances, including calcium and calcium compounds, such as calcium carbonate and calcium sulfate, are especially problematic when they precipitate and build up on the diffuser membrane. Biological growth can also build up and compromise the diffuser efficiency. Diffuser membranes can also be chemically degraded by solvents and various other types of chemicals that may be present in the liquid. This chemical degradation combined with the repeated expansion and contraction of the membrane can weaken the membrane and cause premature structural failure.
One solution to these problems has been to apply a coating to the membrane in order to provide the membrane with a relatively slick surface that resists biological growth and other materials from being deposited thereon. However, the application of the coating is itself not without problems. It is often difficult to establish high bond strengths between the membrane's substrate layer and coating layer, in part, because of the non-adhesive qualities of the coating layer. Various methods have been proposed to address this problem.
One approach is to use an adhesive, bonding or primer layer between the substrate and the coating. By way of example, U.S. Pat. No. 6,759,129 to Fukushi discloses the application of a “bonding” layer between the substrate and coating and U.S. Pat. No. 7,674,514 to Frankel et al. and U.S. Patent Publication No. 2007/0001323 to Kang disclose the application of a “primer” layer between the substrate and coating. Not only does this approach add additional steps, complications and materials in the manufacturing process, but it also results in an increase of the overall product cost.
Another approach is to apply an uncured film to a pre-cured substrate and curing them together in a mold. By way of example, U.S. Pat. No. 7,396,499 to Frankel et al. discloses placing an uncured thin fluoroelastomer film to a pre-cured substrate layer and curing in a high temperature mold. This approach is also disadvantageous in a number of respects. First, because both the substrate and film are in an uncured state, it is not possible to optimize both the curing of the substrate and the bonding of the film to the substrate. The time and temperature requirements for curing differ from those necessary to achieve optimal bonding between the two materials, so either the curing or the bonding must necessarily be compromised. The result is a product that has either an inadequately cured substrate or an inadequate bond between the layers. Second, this approach adds additional steps and complications in the manufacturing process and also results in an increase of the overall product cost. Third, there is no opportunity to clean the substrate because the curing process is interrupted and is only partially completed at the time the film is applied. If contaminants are present, they cannot be removed by solvents or other cleaning processes and can interfere with the bonding to the point of destroying any ability to properly bond the materials together. Fourth, the disclosed fluoroelastomer layer must be applied as a film, thus making it impossible to apply the coating layer in other manners or methods which may be preferable in some cases. For similar reasons, the ability to vary the coating thickness is limited. Finally, this may only be used to produce diffuser membranes that are molded and cannot readily be used to produce diffuser membranes that are extruded.
There are many shortcomings in these existing configurations and the present invention is directed to overcoming one or more, if not all of the above shortcomings.